This invention relates to countermeasures against laser attacks on sensor systems.
Directed energy weapons (xe2x80x9cDEWxe2x80x9d), principally lasers, have become very prominent subjects in current threat assessments. The most developed DEW technology applications utilize various types of lasers as sources of jamming and destructive energy. Laser energy poses a threat to military personnel. Laser energy from ruby or YAG laser, or others, can cause severe damage to unprotected eyes. Also, other sensors on sighting systems can be incapacitated by laser energy. Low power lasers can temporarily incapacitate airborne platform sensors, including air crew eyes, and even cause severe and permanent damage to sensors at distances which greatly exceed the maximum ranges of most conventional air-to-ground weapons. Sensors in all spectral regions are at risk, from visible through the mid and far infrared wavelengths.
The requirements for effective countermeasures are critical. Effective countermeasures must not only protect personnel, sensors and military structures from physical damage, but also they must provide the means to insure mission continuity. That is, they must provide the xe2x80x9clookthroughxe2x80x9d capability in which sensors are protected from temporarily incapacitating or destructive laser radiation by means which will have no significant effect on mission execution.
In addition to low power lasers, somewhat more powerful fixed frequency laser weapon systems have been prototyped and could be fielded by aggressors very quickly. Whether high or low in power, such laser threats call for countermeasures which in general require energy blocking or absorbing devices to filter out potentially harmful radiation.
For example, Roberts and Honeycutt, U.S. Pat. No. 4,673,250 entitled xe2x80x9cCO2 Laser Weapon Countermeasurexe2x80x9d issued Jun. 16, 1987 describes a chemical substance to be dispensed. Having a low diffusion coefficient and a high absorption coefficient for laser radiation, it causes incident laser radiation to bloom, thereby to protect targets. This provides no look-through, however.
Similarly, Karney, U.S. Pat. No. 3,992,628 entitled xe2x80x9cCountermeasure System for Laser Radiation,xe2x80x9d issued Nov. 16, 1976. It defends against laser beam target designators by interposing an aerosol between the laser source and the target to attenuate the beam, but it has no look-through.
Another approach has been Milling, U.S. Pat. No. 3,986,690 entitled xe2x80x9cLaser Defense And Countermeasure System for Aircraft,xe2x80x9d issued Oct. 19, 1976. It discloses a passive defense system using a second skin as a retroreflector. Energy is absorbed when the retroreflective layer is destroyed by a laser. This approach protects equipment and personnel but not their sensors, and has no look-through.
Additionally, various types of filtering technology are currently available and more are under development. These technologies can provide protection and lookthrough capability against low power lasers of known wavelength. However, all fixed wavelength filter technologies exhibit unique strengths and weaknesses. Generally, these characteristics involve trade-offs among achievable optical density, photopic and scotopic transmissivity, and achievable width of field of view. Secondary characteristics such as ballistic robustness, shelf life, weight, and environmental stability are also necessary concerns.
Frequency agile or tunable lasers are now commonly available in laboratories and other facilities, and several types could be fielded as weapons in the neat future. Although fixed wavelength laser countermeasures will continue to be required in many applications to provide safety and hazard protection, it is necessary now to provide countermeasures to broadband, tunable threats. Tunable lasers pose serious inband problems for all optical and electro-optical sensors, ranging from eyes to far infrared (xe2x80x9cFLIRxe2x80x9d) systems. Tunable threats also complicate out-of-band damage problems faced by external optics and aircraft structures.
There are four types or levels of broadband threat: (1) tune before engagement; (2) simultaneous multiple wavelength engagement; (3) discrete wavelength tuning during engagement; and (4) continuous tuning during engagement. The first three of these broadband threats are called quasi-tunable threats. That is, an engagement consists of an attack using one or more unknown but discrete wavelengths. Quasi-tunable threats cannot be countered by fixed frequency filter technologies. This class of threat requires tunable countermeasures.
In the tune before engagement threat, the attacking laser wavelength is unknown but is fixed throughout an engagement. Tuning or wavelength changes would be accomplished between engagements. Similarly, the simultaneous multiple wavelength engagement involves a simultaneous attack by two or more unknown but fixed wavelength lasers. Tuning would be accomplished between engagements. In the discrete wavelength tuning during engagement, the attacking laser energy shifts to discrete (and potentially predictable) wavelengths during an attack. Raman shifting would be typical of this type of threat. Engagement phases would occur at fixed frequencies. Elapsed time between phases would normally exceed one second.
The fourth broadband threat, continuous tuning during engagement, involves an attack by laser energy which is tuned continuously across a wide range of frequencies, with individual wavelength dwell times in the picosecond region. Some current dye lasers, for example, exhibit continuous tunability. Such continuous tuning or frequency change constitutes the most difficult laser threat to counter. This type of threat requires very fast continuous tuning countermeasures with linear predictive capability and/or a totally new class of agile or multiband sensors. It is an object of the present invention to provide such a countermeasure.
As noted supra, two important aspects to any solution to the laser threat are to protect the sensor or structure and provide for mission continuity. Mission continuity becomes particularly difficult to achieve when dealing with broadband threats. Another object of the present invention is to provide such countermeasures which protect the sensor and structure and provide look-through capability for mission continuity.
Another object of the present invention is to detect and identify coherent light at some predetermined threshold energy level, protect against it, and provide lookthrough without significantly altering the mission profile.
An object of the invention is to provide a system with multiple capabilities which are orchestrated and fast operating.
To defend against continuously tunable threats specifically, it is an object of the present invention to provide a system which can include a linear predictive capability with a coherent light detection and identification system. This capability would predict the direction and rate of change of the continuous output of a synchronously pumped die laser.
According to various aspects of the invention, a laser countermeasure system intercepts the leading edge of an incoming pulse of laser energy fast enough to prevent damage to detectors or injury to the human eye. It holds off harmful radiation for a certain amount of time needed for identification of the wavelength of the incoming radiation and tuning of a filtering mechanism, preferably a narrow band filtering mechanism, to block selectively the incoming harmful radiation. Then it restores itself to normal operation with the filtering mechanism in place. In other applications where damage to a sensor is not the concern, the present invention may reduce to automatically identifying the frequency of incident radiation and, in response, automatically interposing one or more filters corresponding to the radiation.